Catalyst for the isomerization of alkanes

ABSTRACT

An improved catalyst particle is disclosed for the conversion of hydrocarbons which comprises a refractory inorganic-oxide support, a Friedel-Crafts metal halide, and a surface-layer platinum-group metal component, wherein the concentration of platinum-group metal component on the surface layer of each catalyst particle is at least 1.5 times the concentration in the central core of the catalyst particle. An isomerization process also is disclosed which is particularly effective for the conversion of C 4  -C 7  alkanes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of prior copending application Ser. No.435,304, filed Nov. 13, 1989, the contents of which are incorporatedherein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved catalyst useful in the conversionof hydrocarbons, particularly for the isomerization of alkanes.

2. General Background

The isomerization of light naphtha is an increasingly important processfor the upgrading of petroleum refiners' gasoline pool. The widespreadremoval of lead antiknock additive from gasoline and the rinsing demandsof high-performance internal-combustion engines are increasing the needfor "octane," or knock resistance, in the gasoline pool. In the earlyyears of lead removal, refiners relied principally upon increasing theoctane of products from catalytic reforming and fluid catalytic crackingoperations. Refiners have largely capitalized on these relativelylow-cost upgrading options, however, and attention has turned in recentyears to upgrading the relatively low-octane high-naphtha component.

Catalyst and process technology for the isomerization of light alkanesis well known in the art. The recent expansion of interest, however, hasled to significant improvements in this technology. Catalyst and processdevelopments have led to lower operating temperatures, wherein productoctane is favored by isomer equilibrium. Substantial reduction in thehydrogen requirement for a stable operation has resulted in asignificant cost reduction, principally through elimination of the needfor a hydrogen-recycle system. Both of the aforementioned developmentshave led toward a predominance of liquid in the isomerization reactorfeed, in contrast to the vapor-phase operation of the prior art.

Catalyst exhibiting a dual cracking and hydrogenation-dehydrogenationfunction are applied widely in the petroleum refining and petrochemicalindustries to the reforming and isomerization of hydrocarbons. Suchcatalysts generally have the cracking function imparted by an inorganicoxide, zeolite, or halogen, with a platinum-group component usuallyimparting the hydrogenation-dehydrogenation function. A catalyst usefulin isomerization must be formulated to properly balance its cracking andhydrogenation-dehydrogenation functions to achieve the desiredconversion over a prolonged period of time, while effectively utilizingthe expensive platinum group metal component.

The performance of a catalyst in isomerization service typically ismeasured by its activity, selectivity, and stability. Activity refers tothe ability of a catalyst to isomerize the reactants into the desiredproduct isomers at a specified set of reaction conditions. Selectivityrefers to the proportion of converted feed reacted into the desiredproduct. Stability refers to the rate of change of activity andselectivity during the life of the catalyst. The principal cause of lowcatalyst stability is the formation of coke, a high-molecular-weight,hydrogen-deficient, carbonaceous material on the catalytic surface.Workers in the isomerization art thus must address the problem ofdeveloping catalysts having high activity and stability, and which alsoeither suppress the formation of coke or are not severely affected bythe presence of coke.

Currently, workers in the art are faced by the problem of developingcost-effective catalysts meeting the above objectives as a concomitantto the low isomerization temperatures and high proportion of liquidreactants described hereinabove.

RELATED ART

Catalysts containing a platinum-group metal component and aFriedel-Crafts metal halide on a refractory inorganic-oxide binder areknown in the art. For example, U.S. Pat. No. 2,900,425 (Bloch et al.)teaches an isomerization process characterized by a catalyst comprisingalumina, a platinum-group metal, and a Friedel-Crafts metal halide.Preferably, aluminum chloride is sublimed on to the alumina-platinumcomposite. Bloch et al. do not disclose a surface-layer platinum-groupmetal; however, the alternative methods of compositing theplatinum-group metal and alumina are all effected before formation ofthe catalyst particle. See also U.S. Pat. Nos. 2,924,629 (Donaldson),teaching an isomerization process; 2,999,074 (Bloch et al.), teaching acatalyst composition of matter; and 3,031,419, teaching a method formanufacturing a catalyst, none of which disclose a surface-layerplatinum-group metal component.

U.S. Pat. No. 3,963,643 (Germanas et al.) teaches a method ofmanufacturing a catalyst useful in the isomerization of paraffins bycompositing a platinum-group metal with alumina and reacting thecomposite with a Friedel-Crafts metal halide and a polyhalo compound.Germanas et al. teaches away from a surface-layer platinum-group metal,noting that: "It is common practice to impregnate the alumina with anaqueous chloroplatinic acid solution acidified with hydrochloric acid tofacilitate an even distribution of platinum on the alumina. . ." (col.2, lines 49-53).

The layering of a platinum-group metal component in a catalyst particlehas been disclosed in the prior art. U.S. Pat. Nos. 3,259,589(Michalko), 3,388,077 (Hoekstra) and 3,931,054 (Lester) teach a catalystpreparation method for providing a subsurface-layer of Group VIII metalor platinum in a catalyst particle. U.S. Pat. No. 3,367,888 (Hoekstra)discloses a method of catalyst preparation wherein the Group VIII metalis deposited on the outer surface of the carrier. U.S. Pat. No.3,651,167 (deRosset) teaches a hydrogenation process characterized by acatalyst comprising a surface-impregnated Group VIII metal or alumina.U.S. Pat. Nos. 3,897,368 (Ohara) and 4,431,750 (McGinnis) disclosecatalyst preparation methods wherein a noble or platinum-group metal isdeposited in high concentration on the surface of the support. U.S. Pat.No. 4,520,223 teaches a method of prepartion of a surface-impregnatednoble metal catalyst useful in a dehydrogenation process. U.S. Pat. Nos.4,716,143 (Imai) and 4,786,625 (Imai et al.) disclose catalystscomprising surface-impregnated platinum, but teach that the catalystsdecrease undersirable side reactions such as isomerization. U.S. Pat.No. 4,556,646 (Bezman) teaches a catalyst with an even radialdistribution of noble metal, and reveals that low penetration of Pd intoa catalyst base increases coking in a hydrocracking reaction. Further,none of these patents disclose the essential Friedel-Crafts metal halidecomponent of the present catalyst.

Thus, no suggestion is offered, in well over 20 years of prior artpertaining to individual components of the present catalyst particle, tocombine a Friedel-Crafts metal halide and a surface-layer platinum-groupmetal component on a refractory inorganic-oxide support. In conformitywith the unpredictability of catalytic effects, the surprising benefitsof this catalyst particle are observed specifically in the context ofmodern, primarily liquid-phase, isomerization operations.

SUMMARY OF THE INVENTION

Objects

It is an object of the present invention to provide a novel catalystparticle, useful particularly for the isomerization of isomerizablehydrocarbons. A corollary object of the invention is to provide aprocess for isomerizing isomerizable hydrocarbons, particularly alkaneshaving from four to seven carbon atoms per molecule.

Summary

This invention is based on the discovery that a catalyst particle havinga Friedel-Crafts metal halide and a surface-layer platinum-group metalon an inorganic-oxide support demonstrates surprising results inincreasing the octane number of C₅ /C₆ naphtha streams.

Embodiments

A broad embodiment of the present invention is a catalyst particlecomprising an inorganic-oxide binder, a Friedel-Crafts metal halide, anda surface-layer platinum-group metal component having a surface-layerconcentration at least 1.5 times that in the central core of eachparticle. Alumina is the preferred inorganic-oxide binder, and aluminumchloride is the preferred Friedel-Crafts metal halide. Platinum is thepreferred surface-layer platinum-group component.

In a preferred embodiment, an organic polyhalo component is added to thecatalyst particle, with carbon tetrachloride being especially preferred.

In another aspect, the invention is a preferred method of preparing thepresent catalyst by surface-impregnating a platinum-group metalcomponent and vaporizing and subliming the Friedel-Crafts metalcomponent onto the inorganic-oxide binder.

In an alternative embodiment, isomerizable hydrocarbons are isomerizedwith the present catalyst particle. The preferred feedstock comprises C₄-C₇ alkanes. These as well as other objects and embodiments will becomeapparent from the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the platinum gradient across the radius of catalystparticles for the Examples described hereinafter. The platinum gradientof a control catalyst particle of the prior art is contrasted with thegradient of catalysts particles of the invention.

FIG. 2 shows comparative product isomer distribution from theisomerization of a light naphtha stream using catalyst particlescontaining surface-layer platinum contrasted with results using catalystparticles containing uniformly distributed platinum. Alternative levelsof platinum on catalyst also are shown in order to show the impact onplatinum requirements of concentrating the platinum in thesurface-layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To reiterate briefly, one embodiment of the present invention is acatalyst particle comprising an inorganic-oxide binder, a Friedel-Craftsmetal halide, and a surface-layer platinum-group metal component havinga surface-layer concentration at least 1.5 times that in the centralcore of each catalyst particle.

Catalyst Support

Considering first the refractory inorganic-oxide support utilized in thepresent invention, it is preferred that the material be a porous,adsorptive, high-surface-area support having a surface area of about 25to about 500 m² /g. The porous carrier material should also be uniformin composition and relatively refractory to the conditions utilized inthe hydrocarbon conversion process. By the term "uniform incomposition," it is meant that the support be unlayered, has noconcentration gradients of the species inherent to its composition, andis completely homogeneous in composition. Thus, if the support is amixture of two or more refractory materials, the relative amounts ofthese materials will be constant and uniform throughout the entiresupport. It is intended to include within the scope of the presentinvention carrier materials which have traditionally been utilized indual-function hydrocarbon conversion catalysts such as: (1) refractoryinorganic-oxides such as alumina, titanium dioxide, zirconium dioxide,chromium oxide, zinc oxide, magnesia, thoria, boria, silica-alumina,silica-magnesia, chromia-alumina, alumina-boria, silica-zirconia, etc.;(2) ceramics, porcelain, bauxite; (3) silica or silica gel, siliconcarbide, clays and silicates including those synthetically prepared andnaturally occurring, which may or may not be acid treated, for exampleattapulgus clay, diatomaceous earth, fuller's earth, kaolin, kieselguhr,etc.; (4) crystalline zeolitic aluminosilicates, such as X-zeolite,Y-zeolite, mordenite, or L-zeolite, either in the hydrogen form or innonacidic form with one or more alkali metals occupying the cationicexchangeable sites; (5) non-zeolitic molecular sieves, such asaluminophosphates or silico-aluminophosphates; (6) spinels such as MgAl₂O₄, FeAl₂ O₄, ZnAl₂ O₄, CaAl₂ O₄, and other like compounds having theformula MO-Al₂ O₃ where M is a metal having a valence of 2; and (7)combinations of materials from one or more of these groups.

The preferred refractory inorganic-oxide for use in the presentinvention is alumina. Suitable alumina materials are the crystallinealuminas known as the gamma-, eta-, and theta-alumina, with gamma-oreta-alumina giving best results. The preferred refractoryinorganic-oxide will have an apparent bulk density of about 0.3 to about1.01 g/cc and surface area characteristics such that the average porediameter is about 20 to 300 angstroms, the pore volume is about 0.1 toabout 1 cc/g, and the surface area is about 100 to about 500 m² /g.

A particularly preferred alumina is that which has been characterized inU.S. Pat. Nos. 3,852,190 and 4,012,313 as a by-product from a Zieglerhigher alcohol synthesis reaction as described in Ziegler's U.S. Pat.No. 2,892,858. For purposes of simplification, such an alumina will behereinafter referred to as a "Ziegler alumina." Ziegler alumina ispresently available from the Vista Chemical Company under the trademark"Catapal" or from Condea Chemie GMBH under the trademark "Pural." Thismaterial is an extremely high purity pseudoboehmite powder which, aftercalcination at a high temperature, has been shown to yield a high-puritygamma-alumina.

The alumina powder may be formed into a suitable catalyst materialaccording to any of the techniques known to those skilled in thecatalyst-carrier-forming art. Spherical carrier particles may be formed,for example, from this Ziegler alumina by: (1) converting the aluminapowder into an alumina sol by reaction with a suitable peptizing acidand water and thereafter dropping a mixture of the resulting sol and agelling agent into an oil bath to form spherical particles of an aluminagel which are easily converted to a gamma-alumina carrier material byknown methods; (2) forming an extrudate from the powder by establishedmethods and thereafter rolling the extrudate particles on a spinningdisk until spherical particles are formed which can then be dried andcalcined to form the desired particles of spherical carrier material;and (3) wetting the powder with a suitable peptizing agent andthereafter rolling the particles of the powder into spherical masses ofthe desired size. This alumina powder can also be formed in any otherdesired shape or type of carrier material known to those skilled in theart such as rods, pills, pellets, tablets, granules, extrudates, andlike forms by methods well known to the practitioners of the catalystmaterial forming art.

The preferred type of carrier material for the present invention is acylindrical extrudate generally having a diameter of about 0.75 to 3.3mm, with about 1.6 mm being especially preferred. The length-to-diameterratio is about 1:1 to 5:1, with about 2:1 being especially preferred.The preferred extrudate particle form of the carrier material ispreferably prepared by mixing the alumina powder with water and suitablepeptizing agents such as nitric acid, acetic acid, aluminum nitrate, andthe like material until an extrudable dough is formed. The amount ofwater added to form the dough is typically sufficient to give a Loss onIgnition (LOl) at 500° C. of about 45 to 65 mass %, with a value of 55mass % being especially preferred. On the other hand, the acid additionrate is generally sufficient to provide 2 to 7 mass % of thevolatile-free alumina powder used in the mix, with a value of 3 to 4mass % being especially preferred. The resulting dough is then extrudedthrough a suitably sized die to form extrudate particles.

The extrudate particles are dried at a temperature of about 150° toabout 200° C., and then calcined at a temperature of about 450° to 800°C. for a period of 0.5 to 10 hours to effect the preferred form of therefractory inorganic-oxide. It is preferred that the refractoryinorganic-oxide comprise substantially pure gamma alumina having anapparent bulk density of about 0.6 to about 1 g/cc and a surface area ofabout 150 to 280 m² /g (preferably 185 to 235 m² /g, at a pore volume of0.3 to 0.8 cc/g).

Platinum-Group Metals

An essential ingredient of the catalyst is the surface-layerplatinum-group metal component. Of the platinum group, i.e., platinum,palladium, rhodium, ruthenium, osmium and iridium, palladium is apreferred component and platinum is especially preferred. Mixtures ofplatinum-group metals also are within the scope of this invention. Thiscomponent may exist within the final catalytic composite as a compoundsuch as an oxide, sulfide, halide, or oxyhalide, in chemical combinationwith one or more of the other ingredients of the composite, or as anelemental metal. Best results are obtained when substantially all ofthis component is present in the elemental state. This component may bepresent in the final catalyst composite in any amount which iscatalytically effective, but relatively small amounts are preferred. Infact, the surface-layer platinum-group metal component generally willcomprise about 0.01 to 2 mass % of the final catalyst, calculated on anelemental basis. Excellent results are obtained when the catalystcontains about 0.05 to 1 mass % of platinum.

An essential feature of the catalyst of the present invention is thatthe platinum-group metal component is concentrated in the surface layerof each catalyst particle. In defining the present invention, a"surface-layer" component has a concentration in the micron surfacelayer of the catalyst particle that is at least 1.5 times theconcentration in the central core of the catalyst particle. Preferably,the surface-layer concentration of platinum-group metal is at leastabout twice the concentration in the central core. As exemplifiedhereinbelow, the surface layer may be 100 or 150 microns deep and thecentral core may be 50% of the volume or 50% of the diameter of theparticle; however, other quantitative criteria are not excluded thereby.

"Layer" is a stratum of the catalyst particle of substantially uniformthickness. The "surface layer" is the layer of the catalyst particleadjacent to the surface of the particle. "Diameter" is defined as theminimum regular dimension through the center of the catalyst particle;for example, this dimension would be the diameter of the cylinder of anextrudate. In the examples presented hereinbelow, platinum concentrationis measured from the surface to the center, or over the radius, of thecatalyst particle. "Central Core" is defined in the present invention asa concentric cylindrical portion of a catalyst particle having a volumeor diameter that is 50% of the volume or diameter; respectively, of thecatalyst particle. As exemplified for the preferred extrudates of thepresent invention, the central core is a concentric cylindrical portionexcluding the surface layer at the ends of the extrudate particles.

The characterization of the platinum-group metal component as a"surface-layer" component is intended to encompass a platinum-groupmetal component gradient upon and within the catalyst support. Theconcentration of platinum-group metal component tapers off inprogressing from the surface to the center of the catalyst particle. Theactual gradient of the platinum-group metal component within thecatalyst particle varies depending upon the manufacturing method used tofabricate the catalyst. However, a substantially greater portion of theplatinum-group metal component is located outside than within thecentral core of the catalyst particle. As previously stated, theconcentration of the platinum-group metal component in the surface layerof the catalyst is at least 1.5 times and preferably twice theconcentration in the central core of the catalyst particle. The gradientof the platinum-group metal component is determined by Scanning ElectronMicroscopy.

The SEM data show the approximate metals content of any one point withinthe catalyst pill, based on the metals distribution profile in relationto the alumina level. The result of each analysis may not be based upona zero point; attempting to integrate a distribution curve is notpossible, and could lead to interpretation errors as the entire curvecould be shifted either up or down. However, the data are useful formaking relative comparisons of metal distributions.

Although the reasons for the surprising improvement in performance arenot well understood, the application of the present catalyst to aprincipally liquid-phase operation is believed to be a factor. Operatingtemperatures required for isomerization of C₄ -C₇ alkanes are lower thanin the past, due to improved catalysts such as the present invention.The proportion of hydrogen in the feed stream is significantly lower inmodern isomerization units, as discussed hereinafter. Both lowertemperatures and lower gas rates result in an increasing proportion ofliquid reactants. The length of diffusion paths within the catalyst isbelieved to be more important for liquid reactants, and thesurface-layer metal component thus permits greater facility of access tocatalyst sites. This theory is supported by evidence that vapor-phaseisomerization operations associated with the prior art do not show thesame benefits from a surface-layer platinum-group metal component.

The platinum-group metal component may be incorporated into the catalystof the present invention by any means suitable to result in asurface-layer component having a concentration in the surface layer ofeach catalyst particle at least 1.5 times that of the central core ofthe particle. Best results are obtained when the platinum-group metalcomponent is surface-impregnated. A preferred method of surfaceimpregnating the platinum-group component is by means of a low-acidimpregnation utilizing a solution of a soluble, decomposable complexcompound of the platinum group component. In general the solvent used inthis impregnation step is selected on the basis of its capability todissolve the desired decomposable complex compound and is a low-acid,preferably aqueous solution. By low-acid it is meant that theimpregnation solution generally has a normality of 2 or less. An HClsolution is preferred, but nitric acid and the like also can be used.

Typical platinum-group compounds which may be employed in preparing thecatalyst of the invention are chloroplantinic acid, ammoniumchloroplatinate, bromoplatinic acid, platinum dichloride, platinumtetrachloride hydrate, platinum dichlorocarbonyl dichloride,dinitrodiaminoplatinum, palladium chloride, palladium chloridedihydrate, palladium nitrate, etc. Chloroplatinic acid is preferred as asource of the especially preferred platinum component. A surface-layerplatinum component may be impregnated onto the catalyst from a solutionof chloroplatinic acid in the absence of strong mineral acids such ashydrochloric and nitric acid.

The platinum group component may be surface impregnated via theformulation of a chemical complex of the platinum-group metal componentwhich is strongly attracted to the refractory oxide support, resultingin the platinum group metal being retained primarily upon the outersurface of the catalyst. Any compound that is known to complex with thedesired platinum-group component and with the metal component of therefractory inorganic-oxide support may be used in the preparation of thecatalyst of the present invention. It has been found that amulti-dentated ligand is very useful in complexing with a platinum groupmetal and the refractory inorganic oxide support., resulting in thesurface-impregnation of the platinum-group metal component.Multi-dentated ligands are compounds that contain more than oneappendage that can bond strongly to the oxide support. Such appendageswould typically comprise carboxylic acids, amino groups, thiol groups,phosphorus groups, or other strongly polar groups of chemicalcomponents.

It is also an aspect of this invention that the multi-dentated ligandcontains a functional group such as --SH or PR₂ (where R is hydrocarbon)that has a high affinity towards the platinum group metal component anda second functional group comprising a carboxylic acid or the likecomponent that can be strongly adsorbed onto the metal oxide support.This preferred property of the multi-dentated ligand effectively insuresthat the platinum group metal component does not penetrate the catalystparticle, by binding strongly with the platinum group metal while alsobinding to the support quickly and strongly. Examples of some usefulmulti-dentated ligands include thiomalic acid, thiolactic acid,mercaptopropionic acid, thiodiacetic acid, thioglycollic acid, andthioproponic acid among others.

It is within the scope of the present invention that the catalyst maycontain other metal components known to modify the effect of theplatinum-group metal component. Such metal modifiers may includerhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc,uranium, dysprosium, thallium, and mixtures thereof. Catalyticallyeffective amounts of such metal modifiers may be incorporated into thecatalyst by any means known in the art.

The composite, before addition to the Friedel-Crafts metal halide, isdried and calcined. The drying is carried out at a temperature of about100° to 300°, followed by calcination or oxidation at a temperature offrom about 375° to 600° C. in an air or oxygen atmosphere for a periodof about 0.5 to 10 hours in order to convert the metallic componentssubstantially to the oxide form.

Halogen Components

Another essential component of the catalyst of the present invention isa Friedel-Crafts metal halide. Suitable metal halides of theFriedel-Crafts type include aluminum chloride, aluminum bromide, ferricchloride, ferric bromide, zinc chloride and the like compounds, with thealuminum halides and particularly aluminum chloride ordinarily yieldingbest results. Generally, this component can be incorporated into thecatalyst of the present invention by way of the conventional methods foradding metallic halides of this type; however, best results areordinarily obtained when the metallic halide is sublimed onto thesurface of the support according to the preferred method disclosed inU.S. Pat. No. 2,999,074, which is incorporated by reference.

In the preferred method, wherein the calcined refractory inorganic-oxidesupport is impregnated with a Friedel-Crafts metal halide component, thepresence of chemically combined hydroxyl groups in the refractoryinorganic oxide allows a reaction to occur between the Friedel-Craftsmetal halide and the hydroxyl group of the support. For example,aluminum chloride reacts with the hydroxyl groups in the preferredalumina support to yield Al--O--AlCl₂ active centers which enhance thecatalytic behavior of the catalyst. Since chloride ions and hydroxylions occupy similar sites on the support, more hydroxyl sites will beavailable for possible interaction with the Friedel-Crafts metal halidewhen the chloride population of the sites is low. Therefore, potentiallymore active Friedel-Crafts type versions of the catalyst will beobtained when the chloride content of the support is the low range ofthe 0.1 to 10 weight percent range.

The Friedel-Crafts metal halide may be impregnated onto the catalyst bysublimation of the Friedel-Crafts metal halide onto the calcined supportunder conditions to combine the sublimed Friedel-Crafts metal halidewith the hydroxyl groups of the calcined support. This reaction istypically accompanied by the elimination of about 0.5 to about 2.0 molesof hydrogen chloride per mole of Friedel-Crafts metal halide reactedwith the inorganic-oxide support. In subliming aluminum chloride, whichsublimes at about 184° C., suitable impregnation temperatures range fromabout 190° C. to 700° C., with a preferable range being from about 200°C. to 600° C. The sublimation can be conducted at atmospheric pressureor under increased pressure and in the presence of absence of diluentgases such a hydrogen or light paraffinic hydrocarbons or both. Theimpregnation of the Friedel-Crafts metal halide may be conductedbatch-wise, but a preferred method for impregnating the calcined supportis to pass sublimed AlCl₃ vapors, in admixture with a carrier gas suchas hydrogen, through a calcined catalyst bed. This method bothcontinuously deposits and reacts the aluminum chloride and also removesthe evolved HCl.

The amount of Friedel-Crafts metal halide combined with the calcined mayrange from about 1 up to 15 mass % to the Friedel-Craftsmetal-halide-free, calcined composite. The final composite containingthe sublimed Friedel-Crafts metal halide is treated to remove theunreacted Friedel-Crafts metal halide by subjecting the composite to atemperature above the sublimation temperature of the Friedel-Craftsmetal halide for a time sufficient to remove from the composite anyunreacted Friedel-Crafts metal halide. In the case of AlCl₃,temperatures of about 400° C. to 600° C. and times of from about 1 to 48hours are sufficient.

In a preferred embodiment of the present invention, the resultantoxidized catalytic composite is subjected to a substantially water-freeand hydrocarbon-free reduction step prior to its use in the conversionof hydrocarbons. This step is designed to selectively reduce theplatinum-group component to the corresponding metal and to insure afinely divided dispersion of the metal component throughout the carriermaterial. Preferably substantially pure and dry hydrogen (i.e., lessthan 20 vol. ppm H₂ O) is used as the reducing agent in this step. Thereducing agent is contacted with the oxidized composite at conditionsincluding a temperature of about 425° C. to about 650° C. and a periodof time of about 0.5 to 2 hours to reduce substantially all of theplatinum-group component to its elemental metallic state. This reductiontreatment may be performed in situ as part of a start-up sequence ifprecautions are taken to predry the plant to a substantially water-freestate and if substantially water-free and hydrocarbon-free hydrogen isused.

An optional component of the present catalyst is an organic polyhalocomponent. In this embodiment, the Friedel-Craftsmetal-halide-containing composite is further treated in contact with apolyhalo compound containing at least 2 chlorine atoms and selected fromthe group consisting of methylene halide, haloform, methylhaloform,carbon tetrahalide, sulfur dihalide, sulfur halide, thionyl halide, andthiocarbonyl tetrahalide. Suitable polyhalo compounds thus includemethylene chloride, chloroform, methylchloroform, carbon tetrachloride,and the like. In any case, the polyhalo compound must contain at leasttwo chlorine atoms attached to the same carbon atom. Carbontetrachloride is the preferred polyhalo compound.

The composite contacts the polyhalo compound preferably diluted in anon-reducing gas such as nitrogen, air, oxygen and the like. Thecontacting suitably is effected at a temperature of from about 100° to600° C. over a period of from about 0.2 to 5 hours to add at least 0.1mass % combined halogen to the composite. The improvement in performanceof the catalyst is not as substantial when the inorganic-oxide supportis first treated with a polyhalo compound before compositing with theFriedel-Crafts metal halide.

The catalyst of the present invention may contain an additional halogencomponent. The halogen component may be either fluorine, chlorine,bromine or iodine or mixtures thereof. Chlorine is the preferred halogencomponent. The halogen component is generally present in a combinedstate with the inorganic-oxide support. Although not essential to theinvention, the halogen component is preferably well dispersed throughoutthe catalyst. The halogen component may comprise from more than 0.2 toabout 15 wt. %, calculated on an elemental basis, of the final catalyst.

The halogen component may be incorporated in the catalyst in anysuitable manner, either during the preparation of the inorganic-oxidesupport or before, while or after other catalytic components areincorporated. For example, the alumina sol utilized to form thepreferred alumina carrier material may contain halogen and thuscontribute at least some portion of the halogen content in the finalcatalyst. The halogen component or a portion thereof also may be addedto the catalyst during the incorporation of other catalyst componentsinto the support, for example, by using chloroplatinic acid inimpregnating a platinum component. Also, the halogen component or aportion thereof may be added to the catalyst by contacting with thehalogen or a compound, solution, suspension or dispersion containing thehalogen before or after other catalyst components are incorporated intothe support. Suitable compounds containing the halogen include acidscontaining the halogen, for example, hydrochloric acid. Alternatively,the halogen component or a portion thereof may be incorporated bycontacting the catalyst with a compound, solution, suspension ordispersion containing the halogen in a subsequent catalyst regenerationstep.

Process

In the process of the present invention, an isomerizable hydrocarboncharge stock, preferably in admixture with hydrogen, is contacted with abed of catalyst particles of the type hereinbefore described in ahydrocarbon isomerization zone. Contacting may be effected using thecatalyst in a fixed-bed system, a moving-bed system, a fluidized-bedsystem, or in a batch-type operation. In view of the danger of attritionloss of the valuable catalyst and of operational advantages, it ispreferred to use a fixed-bed system. In this system, a hydrogen-rich gasand the charge stock are preheated by suitable heating means to thedesired reaction temperature and then passed into an isomerization zonecontaining a fixed bed of the catalyst particle as previouslycharacterized. The isomerization zone may be in a single reactor or intwo or more separate reactors with suitable means therebetween to insurethat the desired isomerization temperature is maintained at the entranceto each zone. Two or more reactors in sequence are preferred to enableimproved isomerization through control of individual reactortemperatures and for partial catalyst replacement without a processshutdown. The reactants may be contacted with the bed of catalystparticles in either upward, downward, or radial flow fashion. Thereactants may be in the liquid phase, a mixed liquid-vapor phase, or avapor phase when contacted with the catalyst particles, with excellentresults being obtained by application of the present invention to aprimarily liquid-phase operation.

Feedstocks

In the group of isomerizable hydrocarbons suitable as feedstock to theprocess of the present invention, alkanes having from 4 to 7 carbonatoms per molecule (C₄ -C₇) are preferred. These may be contained insuch streams from petroleum refining or synthetic-fuel production aslight straight-run naphtha, light natural gasoline, light reformate,light raffinate from aromatics extraction, light cracked naphtha,normal-butane concentrate, field butanes and the like. An especiallypreferred feedstock is light straight-run naphtha, containing more than50% of C₅ and C₆ paraffins with a high concentration of low-octanenormal paraffins; this feedstock is particularly susceptible tooctane-number upgrading by isomerization. The light straight-run naphthaand other feedstocks also may contain naphthenes, aromatics, olefins,and hydrocarbons heavier than C₆. The olefin content should be limitedto a maximum of 10% and the content of hydrocarbons heavier than C₆ to20% for effective control of hydrogen consumption, cracking reactions,heat of reaction and catalyst activity.

It is generally known that high-chloride platinum-alumina catalysts ofthis type are highly sensitive to sulfur- and oxygen-containingcompounds. The feedstock therefore must be relatively free of suchcompounds, with a sulfur concentration generally no greater than 0.5ppm. The presence of sulfur in the feedstock serves to temporarilydeactivate the catalyst by platinum poisoning. Activity of the catalystmay be restored by hot hydrogen stripping of sulfur from the catalystcomposite or by lowering the sulfur concentration in the incoming feedto below 0.5 ppm so that the hydrocarbon will desorb the sulfur that hasbeen adsorbed on the catalyst. Water can act to permanently deactivatethe catalyst by removing high-activity chloride from the catalyst andreplacing it with inactive aluminum hydroxide. Therefore, water andoxygenates that can decompose to form water can only be tolerated invery low concentrations. In general, this requires a limitation ofoxygenates in the feed to about 0.1 ppm or less. The feedstock may betreated by any method that will remove water and sulfur compounds.Sulfur may be removed from the feed stream by hydrotreating. Adsorptionsystems for the removal of sulfur and water from hydrocarbon streams arewell known to those skilled in the art.

Operating Conditions

Hydrogen is admixed with the isomerizable hydrocarbon feed to provide amole ratio of hydrogen to hydrocarbon feed of about 0.01 to 5. Thehydrogen may be supplied totally from outside the process orsupplemented by hydrogen recycled to the feed after separation fromreactor effluent. Light hydrocarbons and small amounts of inerts such asnitrogen and argon may be present in the hydrogen. Water should beremoved from hydrogen supplied from outside the process, preferably byan adsorption system as is known in the art.

Although there is no net consumption of hydrogen in the isomerizationreaction, hydrogen generally will be consumed in a number of sidereactions such as cracking, disproportionation, and aromatics and olefinsaturation. Such hydrogen consumption typically will be in a mol ratioto the hydrocarbon feed of about 0.03 to 0.1. Hydrogen in excess ofconsumption requirements is maintained in the reaction zone to enhancecatalyst stability and maintain conversion by compensation forvariations in feed composition, as well as to suppress the formation ofcarbonaceous compounds, usually referred to as coke, which foul thecatalyst particles.

In a preferred embodiment, the hydrogen to hydrocarbon mol ratio in thereactor effluent is equal to or less than 0.05. Generally, a mol ratioof 0.05 or less obviates the need to recycle hydrogen from the reactoreffluent to the feed. It has been found that the amount of hydrogenneeded for suppressing coke formation need not exceed dissolved hydrogenlevels. The amount of hydrogen in solution at the normal conditions ofthe reactor effluent will usually be in a ratio of from about 0.02 toless 0.01. The amount of excess hydrogen over consumption requirementsthat is required for good stability and conversion is in a ratio ofhydrogen to hydrocarbons of from 0.01 to less than 0.05 as measured atthe effluent of the isomerization zone. Adding the dissolved and excesshydrogen proportions show that the 0.05 hydrogen to hydrocarbon ratio atthe effluent will satisfy these requirements for most feeds. Thecatalyst particles of the present invention show excellent results in aprimarily liquid-phase process operation with reactor-effluenthydrogen-to-hydrocarbon mol ratios of 0.05 or less.

Reactor temperatures will usually range from about 40° to 250° C. Lowerreaction temperatures are generally preferred since the equilibriumflavors higher concentrations of isoalkanes relative to normal alkanes.Lower temperatures are particularly useful in processing feeds composedof C₅ and C₆ alkanes, as lower temperatures favor equilibrium mixtureshaving the highest concentration of high-octane highly branchedisoalkanes. When the feed mixture is primarily C₅ and C₆ alkanes,temperatures in the range of from about 40° to about 150° C. arepreferred. When it is desired to isomerize significant amounts ofbutanes, higher reaction temperatures in the range from about 145° to225° C. are required to maintain catalyst activity.

Reactor operating pressures generally range from about atmospheric to100 atmospheres, with preferred pressures in the range of from 20 to 35atmospheres. Liquid hourly space velocities range from about 0.25 toabout 12 volumes of isomerizable hydrocarbon feed per hour volume ofcatalyst, with a range of about 0.5 to 5 hr⁻¹ being preferred.

The isomerization process also requires the presence of a small amountof an organic chloride promoter. The organic chloride promoter serves tomaintain a high level of active chloride on the catalyst particles, aslow levels are continuously stripped off the catalyst by the hydrocarbonfeed. The concentration of promoter in the combined feed is maintainedat from 30 to 300 mass ppm. The preferred promoter compound is carbontetrachloride. Other suitable promoter compounds include oxygen-freedecomposable organic chlorides such as propyldichloride, butychloride,and chloroform, to name only a few of such compounds. The need to keepthe reactants dry is reinforced by the presence of the organic chloridecompound which may convert, in part, to hydrogen chloride. As long asthe hyrocarbon feed and hydrogen are dried as described hereinabove,there will be no adverse effect from the presence of small amounts ofhydrogen chloride.

Recycle Operation

The isomerization product from the especially preferred light-naphthafeedstock will contain some low-octane normal paraffins andintermediate-octane methylhexanes as well as the desired highest-octaneisopentane and dimethylbutane. It is within the scope of the presentinvention that the liquid product from the process is subjected toseparate and recycle the lower-octane portion of this product to theisomerization reaction. Generally, low-octane normal paraffins may beseparated and recycled to upgrade the octane number of the net product.Less-branched C₆ and C₇ paraffins also may be separated and recycled,along with lesser amounts of hydrocarbons which are difficult toseparate from the recycle. Techniques to achieve this separation arewell known in the art, and include fractionation and molecular sieveadsorption.

The catalyst and process of the present invention reduce theattractiveness of the aforementioned recycle provisions by achievinghigher conversion to desired products without recycle than are realizedby applying the prior art.

The following examples are presented to elucidate the catalyst andprocess of the present invention. These examples are offered asillustrative embodiments and should not be interpreted as limiting theclaims.

EXAMPLE I

A control catalyst of the prior art was prepared in order to demonstratethe advantages of the present catalyst.

An extruded alumina base of the prior art, having a particle diameter ofabout 1600 microns, was vacuum-impregnated in a solution of 3.5 mass %chloroplatinic acid, 2 mass % hydrochloric acid, and 3.5% mass % nitricacid in a volume ratio of 9 parts solution to 10 parts base to obtain apeptized base material having a platinum to base ratio of approximately0.9. The resulting mixture was cold-rolled for approximately 1 hour andevaporated until dry. The composite then was oxidized and the chloridecontent adjusted by contact with an IM hydrochloric acid solution at525° C. at a rate of 45 cc/hour for 2 hours. The composite was thenreduced in electrolytic hydrogen at 565° C. for approximately 2 hoursand was found to contain approximately 0.25 wt. % Pt and approximately 1wt. % chloride. Impregnation of active chloride to a level ofapproximately 5.5 wt. % was accomplished by sublimating aluminumchloride with hydrogen and contacting the catalyst with the sublimatedaluminum chloride for approximately 45 minutes at 550° C.

This catalyst was designated "Catalyst A" and contained approximately0.247 mass % platinum and 5.5 mass % chloride.

EXAMPLE II

"Catalyst B" was prepared in accordance with the invention, withapproximately the same platinum level as Catalyst A in order todetermine the effect of surface-layer platinum. Catalyst B was preparedfrom the same extruded alumina base of the prior art used for Catalyst Aof Example I.

Example B was prepared by evaporative impregnation of the extruded basewith a solution of 3.5 mass % chloroplatinic acid, not containing anyeffective concentration of hydrochloric or nitric acid in order toprevent uniform platinum penetration into the support, in a volume ratioof 9 parts acid to 10 parts base. The resulting mixture was cold-rolledfor approximately 1 hour and evaporated until dry. The composite thenwas oxidized and the chloride content adjusted by contact with a 1Mhydrochloric acid solution at 525° C. at a rate of 45 cc/hr for 2 hours.The composite then was reduced in electrolytic hydrogen at 565° C. forapproximately 2 hours and was found to contain approximately 0.25 wt. %Pt and 1 wt. % chloride. Impregnation of active chloride to a level ofapproximately 5.5 wt. % was accomplished by sublimating aluminumchloride with hydrogen and contacting the catalyst with the sublimatedaluminum chloride for approximately 45 minutes at 550° C.

Catalyst B contained approximately 0.242 mass % platinum and 5.8 mass %chloride.

EXAMPLE III

"Catalyst C" was prepared in accordance with the invention, withapproximately half the platinum level of Catalyst A in order todetermine the effect of surface-layer platinum on platinum requirementsfor the catalyst. Catalyst C was prepared from the same extruded aluminabase of the prior art used for Catalyst A of Example I.

Catalyst C was prepared by vacuum impregnation of the extruded base witha solution of 3.5 mass % chloroplatinic acid, not containing anyeffective concentration of hydrochloric or nitric acid, in a volumeratio of 9 parts acid to 10 parts base. The resulting mixture wascold-rolled for approximately 1 hour and evaporated until dry. Thecomposite then was oxidized and the chloride content adjusted by contactwith a 1M hydrochloric acid solution at 525° C. at a rate of 45 cc/hourfor 2 hours. The composite then was reduced in electrolytic hydrogen at565° C. for approximately 2 hours and was found to contain approximately0.125 wt. % Pt and 1 wt. % chloride. Impregnation of active chloride toa level of approximately 5.5 wt. % was accomplished by sublimatingaluminum chloride with hydrogen on contacting the catalyst with thesublimated aluminum chloride for approximately 45 minutes at 550° C.

Catalyst C contained approximately 0.129 mass % platinum and 5.5 mass %chloride.

EXAMPLE IV

Catalyst particles A, B and C were evaluated by Scanning ElectionMicroscopy (SEM). The purpose of this analysis was to identify thedistribution of platinum across the radius of catalyst particles A, Band C. Six particles each of A and B and three of C were evaluated inorder to provide reliable average data.

The SEM data shows the approximate metals content of any one pointwithin the catalyst pill, as indicated hereinabove, based on the metalsdistribution profile in relation to the alumina level. However, the dataare useful for making relative comparisons of metal distributions.

FIG. 1 shows the relative distribution of platinum level across the800-micron radius of the catalyst particles from the surface to thecenter, relative to the average concentration in the central corerepresenting 50% of the volume of each particle. Catalyst A displayed arelatively even distribution of platinum, with the variation within aregion of the catalyst comparable to the difference between the surfacelayer and central core. Catalysts B and C clearly displayed a relativelyhigh concentration of platinum in the surface layer, with up to about2.5 times the platinum concentration near the surface relative to theconcentration near the center of the catalyst particle.

To quantify the differences between the catalysts of the invention andof the prior art, the platinum concentrations in 100-micron and150-micron surface layers relative to concentrations in the central corewere calculated from the data presented in FIG. 1. Alternative centralcores of 50% of the volume of the catalyst particle (considering theends of the extrudate to be "surface layer") and of 50% of the diameterof the particle were considered. Ratios of concentrations in the surfacelayer and the central core were as follows:

    ______________________________________                                                     Catalyst A                                                                            Catalyst B                                                                              Catalyst C                                     ______________________________________                                        100-micron surface layer:                                                     50% - volume central core                                                                    1.26      1.88      1.67                                       50% - diameter central                                                                       1.31      2.02      1.87                                       core                                                                          150-micro surface layer:                                                      50% - volume central core                                                                    1.27      1.55      1.70                                       50% - diameter central                                                                       1.32      1.66      1.91                                       core                                                                          ______________________________________                                    

EXAMPLE V

Catalysts A, B and C were tested for relative performance inisomerization service. The same feedstock was used for each of thecatalyst tests, and was a hydrotreated light straight-run naphtha havingthe following composition in mass %:

    ______________________________________                                        C.sub.3           0.1                                                         n-Butane          0.3                                                         i-Butane          0.5                                                         n-Pentane         28.3                                                        i-Pentane         18.0                                                        Cyclopentane      3.6                                                         n-Hexane          10.7                                                        2-Methylpentane   13.5                                                        3-Methylpentane   7.5                                                         2,3-Dimethylbutane                                                                              2.2                                                         2,2-Dimethylbutane                                                                              2.3                                                         Methylcyclopentane                                                                              4.7                                                         Cyclohexane       1.8                                                         Benzene           2.3                                                         C.sub.7.sup.+     4.2                                                         Total             100.0                                                       ______________________________________                                    

Once-through isomerization tests were performed at 450 psig, 1.0 liquidhourly space velocity, and 0.3 hydrogen to hydrocarbon mol ratio and aseries of temperatures.

The results are shown in FIG. 2 as curves of the product mass ratios ofisopentane to total pentanes and 2,2-dimethylbutane ("2,2-DMB") to totalhexanes. These ratios are more sensitive tests of catalyst performancethan octane-number measurements, showing the concentration of thehighest-octane isopentane and dimethylbutane isomers in the product witha high degree of precision in contrast to the low reproducibility of themeasurement of product octane.

The comparison of the relative performance of Catalysts A and B show theadvantage of surface-layer platinum. Catalyst B shows an advantage of1-2% for the highest-octane isomers with peak concentrations at lowertemperatures where more favorable equilibrium is obtained. Even thesurface-layer low- platinum Catalyst C shows an advantage over thehigh-platinum control Catalyst A, thus demonstrating that the presentinvention reduces requirements for costly platinum in an isomerizationprocess. The results may be summarized as follows:

    ______________________________________                                                 Peak Research                                                                           Reactor Outlet                                                      Octane Clear                                                                            Temp. at Peak °C.                                   ______________________________________                                        Catalyst A 82.7        149                                                    Catalyst B 83.0        132                                                    Catalyst C 82.8        138                                                    ______________________________________                                    

The temperature at peak Research Octane does not necessarily correspondto the temperature at which the aforementioned C₅ and C₆ isomers reach apeak, due to the influence of other product isomers on Research Octane.However, the conclusions with respect to catalyst performance areconsistent.

We claim:
 1. A catalyst particle comprising a refractory inorganic-oxidesupport, from about 1 to 15 mass % of a Friedel-Crafts metal halide andfrom about 0.01 to 2 mass % on an elemental basis of a surface-layerplatinum-group metal component, wherein the concentration ofplatinum-group metal component on the surface layer of the catalystparticle is at least 1.5 times the magnitude of the concentration ofplatinum-group metal component in the central core of the catalystparticle.
 2. The catalyst particle of claim 1 wherein the surface layeris about 100 microns in depth.
 3. The catalyst particle of claim 1wherein the surface layer is about 150 microns in depth.
 4. The catalystparticle of claim 1 wherein the diameter of the central core is about50% of the volume diameter of the catalyst particle.
 5. The catalystparticle of claim 1 wherein the diameter of the central core is about50% of the diameter of the catalyst particle.
 6. The catalyst particleof claim 1 wherein the refractory inorganic oxide comprises alumina. 7.The catalyst particle of claim 1 wherein the Friedel-Crafts metal halidecomprises aluminum chloride.
 8. The catalyst particle of claim 1 whereinthe platinum-group metal component comprises a platinum component. 9.The catalyst particle of claim 1 further comprising an organic polyhalocomponent in an amount to add at least 0.1 mass % combined halogen tothe catalyst.
 10. The catalyst particle of claim 9 wherein the polyhalocomponent is carbon tetrachloride.
 11. A catalyst particle comprisingalumina, from about 1 to 15 mass % of aluminum chloride and from about0.01 to 2 mass % on an elemental basis of a surface-layer platinumcomponent, wherein the concentration of platinum on the surface layer ofthe catalyst particle is at least 1.5 times the magnitude of theconcentration of platinum in the central core of the catalyst particle.